Without the graph to reference, I can't provide a specific prediction. However, if you know the current moon phase or the moon phase on a specific date close to December 15, you can typically follow a lunar cycle to predict the phase for that date. The lunar cycle is about 29.5 days, and the moon phases progress from new moon to full moon and back.
If you have information about the moon phase on a date before December 15, you can count the days to estimate the phase on December 15. For example, if the full moon was on December 8, then by December 15, the moon would be in a waning phase, likely a waning gibbous or possibly nearing the third quarter.
If you can provide the actual moon phase data leading up to December 15, I’d be able to help you determine which phase is most likely.